U.S. patent number 4,279,873 [Application Number 06/039,892] was granted by the patent office on 1981-07-21 for process for flue gas desulfurization.
This patent grant is currently assigned to A/S Niro Atomizer. Invention is credited to Karsten S. Felsvang, Ove E. Hansen, Elisabeth L. Rasmussen.
United States Patent |
4,279,873 |
Felsvang , et al. |
July 21, 1981 |
**Please see images for:
( Certificate of Correction ) ( Reexamination Certificate
) ** |
Process for flue gas desulfurization
Abstract
SO.sub.2 is absorbed from hot flue gas by spray drying a
Ca(OH).sub.2 -containing suspension in the flue gas. Fly ash is
left in the flue gas which is to be treated in the spray absorption
process, and the powder which is produced by the spray absorption
process and which consequently contains the fly ash and partly
reacted Ca(OH).sub.2 is partially recycled. Operation is controlled
to obtain a temperature of the flue gas after the treatment which
is 8.degree.-20.degree. C. above the saturation temperature of the
flue gas at this stage. The process leads to optimum use of the
Ca(OH).sub.2 used as absorbent and of the neutralization power
inherent in the fly ash. Problems due to sedimentation of the
absorbant before its atomization are avoided.
Inventors: |
Felsvang; Karsten S. (Allerod,
DK), Hansen; Ove E. (Vaerlose, DK),
Rasmussen; Elisabeth L. (Holte, DK) |
Assignee: |
A/S Niro Atomizer (Soborg,
DK)
|
Family
ID: |
8110990 |
Appl.
No.: |
06/039,892 |
Filed: |
May 17, 1979 |
Foreign Application Priority Data
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May 19, 1978 [DK] |
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2237/78 |
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Current U.S.
Class: |
423/244.07;
423/244.08; 423/243.08; 423/244.02 |
Current CPC
Class: |
B01D
53/501 (20130101) |
Current International
Class: |
B01D
53/50 (20060101); B01D 53/50 (20060101); C01B
017/00 () |
Field of
Search: |
;423/244A,244R,242A,242R,243 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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96138 |
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Oct 1958 |
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CS |
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2304496 |
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Aug 1974 |
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DE |
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2419579 |
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Nov 1974 |
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DE |
|
2550488 |
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May 1977 |
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DE |
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1333635 |
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Oct 1973 |
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GB |
|
Primary Examiner: Vertiz; O. H.
Assistant Examiner: Heller; Gregory A.
Attorney, Agent or Firm: Schuyler, Banner, Birch, McKie
& Beckett
Claims
We claim:
1. A process for desulfurization of fly ash-containing flue gas
from the combustion of coal, comprising the steps of:
(a) atomizing an aqueous feed suspension comprising Ca(OH).sub.2
and fly ash into the hot flue gas coming from a boiler without
previous removal of fly ash therein, the amount of said suspension
and the water content thereof being adjusted to obtain a
temperature of the desulfurized flue gas being 8.degree.-40.degree.
C. above the saturation temperature of said gas, thereby
simultaneously drying the atomized droplets of suspension and
reducing the SO.sub.2 content of the flue gas;
(b) collecting a fly ash-containing dry powder comprising
conglomerates of fly ash originating from said feed suspension with
materials formed by the drying and SO.sub.2 absorption step;
(c) preparing an aqueous suspension from a recycled portion of said
fly ash-containing dry powder and disposing of the remaining
portion; and
(d) adding freshly slaked lime to the last-mentioned suspension as
makeup Ca(OH).sub.2 to produce a feed suspension to be atomized in
step (a).
2. A process for the desulfurization of hot, fly ash-containing
flue gas from the combustion of sulfur-containing fuel, said
process comprising the steps of:
(a) preparing an aqueous feed suspension comprising calcium
hydroxide and fly ash;
(b) atomizing said feed suspension in a stream of said hot, fly
ash-containing flue gas in a drying chamber to effect drying of the
resulting atomized droplets and absorption of sulfur dioxide in
said flue gas;
(c) recovering a dry fly ash-containing powder comprising
conglomerates of fly ash originating from the feed suspension with
materials resulting from the drying and sulfur dioxide absorption
step, and
(d) recycling a portion of the fly ash-containing powder recovered
in step (c) for use in preparation of said aqueous feed
suspension;
said drying and sulfur dioxide absorption being effected while
maintaining the temperature of the flue gas effluent from said
drying chamber at from about 8.degree. to about 40.degree. C. above
the adiabatic saturation temperature of said gas by controlling the
amount of feed suspension forwarded to said drying chamber and the
total solids content of said feed suspension, in response to the
amount, temperature and moisture content of the flue gas feed in
said drying chamber.
3. A process as claimed in claim 1 or 2, wherein the amount of the
recycled portion of said fly ash-containing powder is sufficient to
obtain a suspension having after addition of makeup calcium
hydroxide a total solids content of 30-55% by weight.
4. A process as claimed in claims 1 or 2, wherein the flue gas, the
sulfur dioxide content of which is to be reduced, is divided in an
adjustable manner into two streams, one of them being fed to air
dispersing means disposed in the upper part of a drying chamber of
a spray drier and being dispersed around a rotary atomizer wheel
hanging in the drying chamber, while the other stream is introduced
into the central part of the drying chamber and is dispersed under
the atomizer wheel.
5. A process as claimed in claims 1 or 2, wherein the calcium
hydroxide-containing suspension contains an agent for increasing
the solubility in water of calcium hydroxide.
6. The process of claim 2 wherein the recycled portion of said fly
ash-containing powder is mixed with water to form a first
suspension and said feed suspension is prepared by adding to said
first suspension fresh makeup calcium hydroxide in the form of an
aqueous calcium hydroxide paste produced by slaking calcium
oxide.
7. The process of claims 1 or 2 wherein the molar ratio per unit
time between the total reactive calcium hydroxide in said feed
suspension and the sulfur dioxide in said flue gas feed is
maintained in the range of from 0.3-2.0 by adjusting the
composition and amount of said feed suspension and wherein from
about 80 to about 98% of the sulfur dioxide is removed from said
flue gas.
8. The process of claim 1 or 2 wherein the recycled portion of said
fly ash-containing powder comprises powder recovered directly from
said drying chamber.
9. The process of claim 1 or 2 wherein the recycled portion of said
fly ash-containing powder comprises powder recovered directly from
said drying chamber and powder recovered from the flue gas after
leaving the drying chamber.
10. The process of claim 2 wherein said hot, fly ash-containing
flue gas is from the combustion of coal in a boiler and said flue
gas has had no prior removal of fly ash therefrom.
11. The process of claim 2 wherein said feed suspension is prepared
by mixing fresh makeup calcium hydroxide with said recycled
powder.
12. The process of claim 2, wherein the part of said dry powder
recovered from the flue gas after leaving said drying chamber is
recovered by means of a particulate removal device.
13. The process of claim 12 wherein said particulate removal device
is a bag filter unit.
Description
FIELD OF THE INVENTION
The present invention is directed to an improved process for
desulfurization of flue gas from the combustion of
sulfur-containing fuel, in which process an aqueous calcium
hydroxide-containing suspension is atomized in a hot flue gas
stream, whereby the atomized suspension is dried to form a powder
while a substantial amount of the SO.sub.2 is simultaneously
absorbed, and in which process the produced powder is partly
recycled to the calcium hydroxide-containing suspension.
BACKGROUND OF THE INVENTION
Processes of the above-indicated type are well known in the art,
viz. processes in which sulfur dioxide and sulfuric acid formed by
oxidation thereof are fixed as sulfite and sulfate in a dry powder
which results when a sulfur dioxide-containing flue gas is used as
drying gas in a spray drying process in which the liquid which is
atomized is a solution of a soluble strong base or a suspension of
calcium hydroxide.
Such a process is described e.g. in U.S. Pat. No. 3,932,587.
According to this specification the basic liquid is an aqueous
solution or suspension containing at the most 40% by weight alkali
metal carbonate or hydrogen carbonate, preferably sodium carbonate
and/or sodium hydrogen carbonate. Due to the fact that the time is
very short in which each atomized droplet of the basic liquid can
react with the sulfur dioxide, it has hitherto been regarded as
necessary (although the use of a calcium hydroxide suspension as
basic liquid has been suggested) that the basic liquid should
comprise a solution of a very soluble and very reactive basic
substance such as e.g. sodium hydroxide or, as mentioned above,
sodium carbonate. Indeed, a certain reaction between the sulfur
dioxide and the particles formed by the atomization can take place
also after the latter have been dried to a substantially anhydrous
powder, but far the dominating part of the absorption reaction
takes place while a liquid phase is still present in the atomized
particles.
The water soluble substances, e.g. sodium carbonate, coming into
consideration in the known processes of this type, are in many
places so expensive, and the resulting sulfite- and
sulfate-containing powder has so limited market, that the use of
said soluble substances mainly comes into consideration in
connection with a regeneration stage in which the sulfite- and
sulfate-containing powder from the spray absorption-drying process
is regenerated to carbonate or hydroxide. Regeneration in
connection with the use of alkali metal hydroxide or carbonate as
absorbents can, moreover, be necessary because disposal of the
resulting sulfite-containing material may cause problems, as there
is a risk that the material disposed of may contaminate subsoil
water and streams due to the high solubility of the material.
However, such regeneration processes which i.a. are described in
the above U.S. specification require an extensive plant and a
complex operation, which has impeded the industrial utilization
thereof.
Therefore, it is desired to provide a process of the type stated in
the first paragraph of the present specification, in which process
a basic material, viz. Ca(OH).sub.2, is used, which is so
inexpensive and readily available that regeneration of the sulfite-
and sulfate-containing product can be dispensed with, and in which
the absorbent is utilized effectively and results in a powder which
is easy to handle and discharge.
Several processes are known for desulfurization of flue gas, in
which processes the flue gas is scrubbed with an aqueous basic
liquid, e.g. in a scrubbing tower.
By the processes of this last-mentioned type, the basic liquid is
not dried to produce a powder but leaves the reaction zone as a
sulfite- and sulfate-containing liquid which is possibly recycled
and/or regenerated. Processes of this last-mentioned type are e.g.
disclosed in Danish specification No. 123 337, Swedish published
patent application No. 371 368, U.S. Pat. No. 3,533,748 and DE-OS
Nos. 2 304 496, 2 419 579 and 2 550 488.
The above-mentioned prejudice that calcium hydroxide is not
suitable for processes in which the SO.sub.2 absorption and spray
drying of the absorbent take place simultaneously has been
prevailing in spite of the fact that already in 1960 it was
suggested in Czechoslovakian specification No. 96 138 to use a
calcium hydroxide suspension as absorbent. In said specification it
was suggested to improve the conversion of the calcium hydroxide by
partially recycling the resulting powder to the suspension of
Ca(OH).sub.2 to be atomized. However, this process has not found
any substantial application, probably because the Ca(OH).sub.2
consumption has been unacceptable high in spite of the
recycling.
In the method disclosed in said Czechoslovakian specification the
fly ash contained in the flue gas is collected before the gas is
contacted with the atomized calcium hydroxide suspension. It is not
specified which temperature and humidity shall be obtained for the
flue gas at the end of the treatment.
SUMMARY OF THE INVENTION
It has now be found that it is possible to perform a process as the
one stated in the first paragraph of the present specification,
i.e. a process of the type dealt with in the above-cited
Czechoslovakian specification, using substantially less calcium
hydroxide and still obtaining a sufficient SO.sub.2 absorption,
while the amount of substance which is recycled is kept at a low
level acceptable when performing the process on a commercial
scale.
This is achieved by a method which according to the invention is
characterized in that fly ash present in the flue gas is only
separated from the latter after the absorption and spray drying
process, and is partly recycled together with a part of the powder
resulting from the drying process to the step in which the calcium
hydroxide-containing suspension is manufactured, and the ratio
between on one side the amount of the calcium hydroxide-containing
suspension and the content of dry matter thereof and, on the other
side the temperature and moisture content of the flue gas, is
adjusted to obtain a temperature of the flue gas after the
treatment which is 8.degree.-40.degree. C. above the adiabatic
saturation temperature (as to water) of said gas.
The reason why a specially efficient SO.sub.2 removal is obtained
by the process according to the invention cannot solely be
explained by the fact that, in contrast to the method according to
said Czechoslovakian specification, utilization of the absorption
ability (known per se from e.g. DE-OS NO. 2 638 581) is obtained
for the total amount of fly ash. The fact that fly ash is only
separated after the absorption and spray drying process and is
partly recycled for the production of the calcium
hydroxide-containing suspension involves special advantages which
are important for the efficiency of the absorption process. When
the fly ash together with the powder produced in the absorption and
spray drying process is recycled to the preparation of the calcium
hydroxide suspension, the fly ash has a suspending effect on the
particles of calcium hydroxide, which means that flocculation of
the calcium hydroxide particles is substantially avoided, which
flocculation would have been important if the calcium hydroxide
suspension without recycling of fly ash had been diluted to obtain
a sufficiently low viscosity to enable atomization. In this way the
recycled particles of fly ash have the effect that the calcium
hydroxide particles will be more evenly distributed and have a
greater surface in the droplets formed by the atomization of the
suspension. This greater surface gives a more complete reaction
with the sulfur dioxide.
The fly ash particles remain, just as the particles produced by the
absorption and spray drying process, substantially intact in the
calcium hydroxide-containing suspension to be spray dried, and in
the spray drying both of these types of particles will form nuclei
in the droplets, on which nuclei the substantially smaller
particles of fresh added calcium hydroxide is present. These
last-mentioned calcium hydroxide particles will in this position
have better conditions for reaction with the sulfur dioxide than if
they e.g. were positioned in the central part of a particle
consisting solely of small calcium hydroxide particles. Recycled
powder enhances thereby the absorption by acting as carrier for the
fresh added small calcium hydroxide particles, and for this purpose
the fly ash particles are especially efficient because they, at
least when they originate from a coal-fuelled boiler, have a
substantially smaller size than the particles formed by the
absorption and spray-drying process, which small size has proven to
be optimum when the particles are to perform said carrier
function.
A determining feature for obtaining a sufficient absorption using a
relatively small amount of calcium hydroxide is, moreover, that the
drying process is operated using such conditions that the flue gas
after the treatment has a temperature which is 8.degree.-40.degree.
C. above the saturation temperature of the treated gas. It has been
found that when the latter conditions are fulfilled a substantially
better absorption is achieved than when the temperature is outside
this range. If the conditions are so that the gas after the
treatment has a temperature which is more than 40.degree. C. above
its saturation temperature, the resulting powder will have a
relatively high content of non-reacted calcium hydroxide, probably
bacause the drying under these conditions is performed so fast that
the period is too short in which the particles have a sufficient
moisture content to enable a substantial reaction with the sulfur
dioxide. Also by using temperatures lower than said range
unsatisfactory results are, however, obtained. This is due mainly
to the fact that in these cases a relatively large amount of
calcium carbonate is formed, which has a somewhat lower reactivity
to sulfur dioxide than calcium hydroxide. This production of
carbonate will of course be especially adverse in a process using
partial recycling as the present one.
It is surprising and quite unpredictable that a lower temperature
limit exists which is caused by carbonate production.
It is observed that it is of substantial importance, although the
process includes recycling of particles containing unreacted
calcium hydroxide, that optimum reaction conditions prevail during
the SO.sub.2 absorption process, as otherwise the costs as regards
apparatus as well as operation thereof will be increased.
A preferred embodiment of the process according to the invention is
characterized in that the mixture of fly ash and powder produced by
the process which is removed from the flue gas after the drying and
absorption process is, for use in the preparation of the calcium
hydroxide-containing suspension, suspended in water and only
afterward brought into contact with makeup calcium hydroxide in the
form of an aqueous Ca(OH).sub.2 paste produced by slaking of
CaO.
This embodiment presents advantages as compared to what is achieved
when the dry recycled powder consisting of fly ash and particles
formed by the absorption and spray drying process are mixed with a
previously diluted suspension of makeup calcium hydroxide. The said
preferred process results in a better utilization of the alkalinity
of the fly ash, as a better extraction is obtained with water than
with a calcium hydroxide suspension, and, moreover, one achieves a
better utilization of the freshly supplied calcium hydroxide, as by
ensuring that the recycled particles including fly ash are
saturated with water before being brought into contact with the
calcium hydroxide suspension, one obtains that the calcium
hydroxide only to a very small extent penetrates into the interior
of the particles where it would not be in an optimum position to
exert its SO.sub.2 absorbing effect. On the contrary the calcium
hydroxide particles will during the drying of the atomized droplets
be deposited on the surface of the recycled particles where they
have particylarly good possibilities of reacting with the sulfur
dioxide. By this embodiment one furthermore avoids diluting the
aqueous paste of calcium hydroxide formed by slaking of quicklime
which is an advantage as such a dilution results in a certain
agglomeration of the fine calcium hydroxide particles formed by
slaking. When the said calcium hydroxide paste, on the contrary, is
mixed with a suspension of the recycled particles, said suspension
having about the same total solids content as the slaked lime paste
but a substantially lower viscosity than this, such agglomeration
is avoided and simultaneously one obtains a so substantial
reduction of the viscosity of the calcium hydroxide paste that it
becomes pumpable and is suitable for spraying. Said agglomeration
of the calcium hydroxide particles is of course undesirable as it
reduces the surface area of the particles and thereby impairs the
reaction conditions for SO.sub.2 absorption. In this embodiment the
fresh makeup calcium hydroxide paste is preferably added to the
suspension of recycle particles only immediately before the said
suspension is atomized, avoiding thereby problems caused by crystal
growth and sedimentation.
However, satisfactory results can also be achieved by adding the
recycled particles to the aqueous Ca(OH).sub.2 paste without any
preceeding suspension of the particles in water, which addition may
be carried out simultaneous with or after the dilution of said
Ca(OH).sub.2 paste. Also in this way a substantial part of the
advantages mentioned in the paragraph immidiately above may be
achieved as the recycled fly ash particles have a certain
redispersing action on calcium hydroxide agglomerates which might
have been formed before the addition of the recycled particles.
Fly ash has such a particle size that its presence in the recycled
material contributes to a large extent to the obtainment of a high
solids content in the suspension to be atomized without increasing
the viscosity of said suspension to an inadmissible extent. This
high solids content involves that a high SO.sub.2 absorption may be
obtained while at the same time the water evaporation is kept on a
suitable low level. This in turn means that cooling and increase in
water content of the flue gas will be less extensive, which is in
itself an advantage, and moreover it becomes easier to meet the
above requirements as to the temperature of the treated flue gas in
relation to the saturation temperature of the latter.
In addition to the mentioned advantages of the process as compared
to the related known processes, in particular the process disclosed
in the above mentioned Czechoslovakian patent, a particular
advantage of the process according to the invention is that is
renders superfluous the use of a device for removing fly ash prior
to the absorption and spray drying step.
Another embodiment is according to the invention characterized in
that the composition and amount of the calcium hydroxide suspension
is adjusted so that the molar ratio between calcium hydroxide and
sulfur dioxide introduced into the spray dryer per time unit is
0.3-2.0 and the sulfur dioxide absorption 80-98%. When operating
under such conditions, one obtains a satisfactory sulfur dioxide
removal and at the same time the quantity of absorption material to
be atomized is kept within economically acceptable limits.
A further preferred embodiment is according to the invention
characterized in that the calcium hydroxide containing liquid
contains an agent for increasing the solubility in water of the
calcium hydroxide, preferably sodium chloride. It has been found
that one obtains thereby a substantial improvement of the sulfur
dioxide absorption. In tests where no recycling of the produced
powder took place, an increase of 6.7% of the sulfur dioxide
absorption was obtained by adding sodium chloride in an amount
corresponding to about 0.5% of the quantity of liquid atomized. The
provision of an optimum amount of sodium chloride in the basic
liquid will often require no particular measures, as the water
available for the formation of the suspension of recycled material
will often contain a suitable amount of sodium chloride.
BRIEF DESCRIPTION OF THE DRAWINGS
The process according to the invention is illustrated further in
the following with reference to the drawing where
FIG 1 is a very simplified flow sheet of an embodiment of the
process,
FIG. 2 is a very simplified flow sheet of an other embodiment of
the process,
FIG. 3 shows in partial section a spray drier particularly suitable
for the carrying out of the process on a commercial scale, and
FIG. 4 shows a section marked IV--IV in FIG. 3.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
In the drawing, 1 is a suspension tank to which are supplied water
and recycled particles consisting of fly ash and particles formed
by the absorption and spray drying process described below, as
indicated by the conduits 2 and 3, respectively. The suspension
formed in tank 1 is fed through conduit 4 to a mixing tank 5. To a
slaker 6 is fed quicklime through duct 7 and water through conduit
8. From slaker 6 the Ca(OH).sub.2 suspension is fed to the mixing
tank 5 through conduit 9. The ratio between the two suspensions fed
to this tank is in each case adjusted according to the sulfur
dioxide content of the flue gas, i.e. according to the sulfur
content of the fuel used in the period in question, and according
to the temperature of the gas.
The amount of recycled powder fed to the suspension tank 1 through
conduit 3 may vary within wide limits and represents e.g. between
10 and 90%, preferably between 30 and 70%, and in typical cases
about 50% of the amount of powdered material including fly ash
produced by the spray drying in question.
The slaking taking place in the slaker 6 results in calcium
hydroxide having the form of very fine particles uniformly
distributed in the paste resulting from the slaking. When this
paste is in the mixing tank 5 mixed with the suspension from the
suspension tank 1 no substantial flocculation or agglomeration of
the fine calcium hydroxide particles takes place as would be the
case if the calcium hydroxide paste had been diluted with water,
which would at first sight had been most obvious. The presence of a
great amount of particles, in particular fly ash, in the suspension
wherein the freshly produced calcium hydroxide paste is mixed has
surprisingly been found to have a markedly stabilizing effect so
that the fine calcium hydroxide particles remain in suspension.
In order to achieve an optimum utilisation of this effect it has
according to the invention proved advantageous to recycle a
sufficient amount of the mixture of fly ash and powder formed by
the absorption and spray drying process to obtain a suspension
having after addition of makeup calcium hydroxide a total solids
content of 30-55% by weight.
From the mixing tank 5 the mixture of Ca(OH).sub.2 suspension and
suspension of recycled material is fed immediately through conduit
10 to a spray drying plant 11 wherein it is atomized, preferably
using a rotary atomizer wheel 12 preferably of the abrasion
resistant type as described in British Pat. No. 1,276,000.
The hot sulfur dioxide-containing flue gas to be purified is fed
through duct 13 through which, in the embodiment shown in FIG. 1,
the total amount of gas is fed to a roof air disperser 14 in the
spray dryer.
The quantity of water fed to tank 1, the quantity of powder
recycled thereto and the ratio between the supplies through 4 and
9, and the amount of suspension fed to the atomizer wheel are
adjusted in view of the quantity of flue gas and its sulfur dioxide
content and of the temperature of said gas, so that the molar ratio
between supplied calcium hydroxide (including the amount contained
in the recycled material in the suspension) and the sulfur dioxide
per unit of time is between 0.3 and 2.5, and so that the flue gas
after drying will have a temperature and moisture content
corresponding to the above stated requirements, viz. a temperature
from 8.degree. to 40.degree. C. above the sat. temp. Such
calculations are a matter of routine to those skilled in the
art.
In the spray drier the sulfur dioxide-containing gas is flowing
from the air disperser 14 toward the lower part of the plant and is
thereby brought into close contact with the droplets of calcium
hydroxide-containing suspension ejected from the atomizer wheel 12.
The temperature of the flue gas will at the inlet of the spray
drier be in typical cases 120.degree.-190.degree. C., preferably
140.degree.-160.degree. C., and while the SO.sub.2 is absorbed by
the liquid droplets and reacts with the calcium hydroxide dissolved
and suspended therein, a strong evaporation of the water contained
in the droplets will take place. The droplets containing one or
more fly ash particles or particles formed by spray drying which
are recycled through conduit 3 will during part of the drying
process have a nucleus constituted of one or more of these recycled
particles, on the surface of which are positioned small particles
of the freshly fed calcium hydroxide, which small particles in this
position will have particularly good conditions for reacting with
the sulfur dioxide of the flue gas.
The drying and chemical reaction of the atomized droplets result in
a powder containing calcium sulfite and, further, some calcium
sulfate and calcium carbonate while the remainder is mainly
unreacted calcium hydroxide and fly ash.
Part of this powder is removed together with a further amount of
fly ash through the bottom of the spray drier via conduit 15, while
the remaining part together with the flue gas partially free from
sulfur dioxide leaves the spray drier through duct 16. To this duct
is preferably connected a duct (not shown) for supplying hot, non
purified flue gas to be mixed with the purified gas with a view to
increasing the temperature of the latter.
The duct 16 leads to a bag filter unit 17 where the flue gas is
freed from entrained particles which are removed through conduit
18.
In large industrial plants it may be considered to use instead of
the bag filter unit 17 an electrostatic pecipitator or another
means for removing dust from a gas stream.
From unit 17 the purified gas is led via a pipe 19 to a stack 20
where its temperature will be sufficient to enable the gas to
disperse into the atmosphere and to avoid condensation in the
immediate proximity of the stack.
The powder removed through conduits 15 and 18 containing in
addition to the particles formed by the absorption and spray drying
process the essential part of the fly ash content of the flue gas,
is led to conduit 21 from where it is partly recycled via 3 and
partly removed through a conduit 22.
In FIG. 2 which as indicated illustrates an amended embodiment of
the process according to the invention, the individual parts of the
plant have been given the same numbers as corresponding parts in
FIG. 1. Also in this embodiment the slaker 6 is fed with quickline
through duct 7 and with water through conduit 8. The lime slaked
with a surplus of water is fed to the mixing tank 5 through conduit
9. In the mixing tank 5 a dilution with water takes place (as
indicated on the drawing) and recycled powder is added through
conduit 3. The suspension prepared in this way is via conduit 10
fed to the spray drying plant 11 and the rest of the process is
performed as described in connection with FIG. 1.
In large industrial plants it will be appropriate, instead of the
spray drier 11 shown in FIG. 1 and FIG. 2, to use a spray drier of
the type shown in FIG. 3. In this device the flue gas, the sulfur
dioxide content of which is to be reduced, is divided in an
adjustable manner into two streams, one of them being fed to a roof
air disperser 30 through which it is dispersed over a rotary
atomizer wheel 32 hanging down in a drying chamber 31, while the
other stream is through a duct 34 led to the central part of the
drying chamber 31 and is dispersed under the atomizer wheel. In
this ambodiment the dispersion of this latter stream takes place by
means of a disperser 35 which by means of vanes imparts to the air
stream a rotary upward movement. A suitable adjustment of the ratio
between the stream sent to the roof air disperser 30 and the stream
dispersed via 35 makes it possible to achieve optimum contact
between the atomized liquid droplets and the gas to be purified,
which is of particular importance in the process in question, as
the ratio between gas and liquid is much greater in this process
than it is the case in the conventional spray drying processes.
Moreover the spray drying plant shown in FIG. 3 makes it possible
to achieve an efficient operation of the spray drying process even
when the flow rate of the flue gas is subject to substantial
variations as is the case in the treatment of power plant flue gas.
Said flexibility of this plant is mainly due to the fact that the
ratio may be adjusted between the gas introduced through the roof
disperser 30 and the gas introduced through the disperser 35.
The purified gas and the entrained particles leave the spray drying
chamber 31 through duct 36 and part of the powder formed by the
spray drying and of the fly ash is removed at the bottom of the
chamber through an outlet positioned af 36.
In order to explain FIGS. 3 and 4, it should be remarked that above
the approximately horizontal parts of ducts 34 and 36, provision is
made of screens 38 and 39, respectively, in order to prevent powder
deposits in these ducts.
The invention will be illustrated further by the following
examples.
EXAMPLE I
A pilot plant designed as the one outlined in FIG. 1 was used. The
stream of flue gas led through duct 13 amounted to 15,800 kg/hour
and had a sulfur dioxide content of 1500 ppm (based on volume) and
a fly ash content of about 5 g/m.sup.3. Slaked lime was fed from
the slakes 6 to the mixing tank 5 in an amount corresponding to
about 70 kg Ca(OH).sub.2 /hour. The tank 1 received per hour 210 kg
of recycled powder having a calcium hydroxide content of 4% from
which tank the aquous suspension was fed to the mixing tank 5.
The spray drier 11 had the following dimensions: diameter 3.3 m,
height of the cylindrical part 2.2 m, cone angle 60.degree. C.
The temperature of the flue gas fed through 13 was 156.degree. C.
and its temperature when leaving the spray drier was 76.degree. C.,
which can be calculated as being 23.degree. C. above the saturation
temperature of the gas.
The powder recycled through conduit 3 had, as already mentioned, a
content of non-reacted calcium hydroxide of 4% by weight, so that
it can be calculated that the ratio between the total amount of
calcium hydroxide fed to the spray drier and the amount of sulfur
dioxide supplied was, on a molar basis, 1.4.
Under these conditions an absorption of 84% of the sulfur dioxide
contained in the flue gas was obtained.
COMPARATIVE EXAMPLE 1
The procedure was the same as in Example I except that no recycling
through conduit 3 took place, while the amount of fresh calcium
hydroxide fed to the mixing tank was increased so that the total
amount of calcium hydroxide fed to the atomizer wheel per time unit
was the same as in Example I, and consequently, in this comparative
example the ratio between calcium hydroxide and sulfur dioxide,
calculated on molar basis, was also 1.4. In this case the sulfur
dioxide absorption was only 67%. Thus it appears that the recycling
of powder containing fly ash and particles originating from the
absorption and spray drying process, due to the above-described
carrier effect and the utilization of fly ash alkalinity results in
a substantially better sulfur dioxide absorption although the total
amount of calcium hydroxide present in the absorption process is
the same.
The reproducibility of the experiments reported in the above
example and comparative example is so high that the absorption
increase obtained by including in the absorbent material recycled
particles of fly ash and spray dried particles is significant.
COMPARATIVE EXAMPLE 2
The procedure was the same as in Example I except that the
temperature of the flue gas led through duct 13 was 180.degree. C.,
which resulted in the temperature of the effluent gas being
48.degree. C. above the saturation temperature. In this case the
sulfur dioxide absorption was only 70%.
COMPARATIVE EXAMPLE 3
The procedure was the same as in Example I except that the fly ash
was removed from the flue gas before said gas was led through duct
13 to the spray drier. In this case the powder removed through 15
and 18 contained only immaterial amounts of fly ash and its content
of calcium hydroxide was 7% by weight. The amount of powder
recycled to the tank 1 was therefore reduced correspondingly so
that the total amount of recycled calcium hydroxide was the same as
in Example I. In this case a sulfur dioxide absorption of 76% was
obtained, thus substantially less than the absorption achieved in
Example I where fly ash was present during the absorption process
and participated in the recycling.
EXAMPLE II
The procedure was the same as in Example I but in this case the
amount of flue gas was 21,700 kg/hour and the sulfur dioxide
content was 1330 ppm, based on volume.
The suspension of slaked lime was fed in an amount corresponding to
112 kg Ca(OH).sub.2 /hour. The recycled amount of powder was 160
kg/hour and this powder contained about 9% of calcium
hydroxide.
Through conduit 22 a quantity of 252 kg of powder/hour was removed
having a calcium hydroxide content of 9%.
The temperature of the flue gas was 146.degree. C. when entering
the spray drier and when leaving the spray drier it was 76.degree.
C., which is about 21.degree. C. above the saturation
temperature.
The molar ratio between calcium hydroxide and SO.sub.2 was in this
case 1.8 and a sulfur dioxide absorption of 91% was obtained.
EXAMPLE III
The procedure was the same as in Example I but in this case the
amount of fly ash-containing flue gas was 20,800 kg/hour with a
sulfur dioxide content of 1320 ppm. The amount of calcium hydroxide
suspension corresponded to 125 kg Ca(OH).sub.2 per hour.
An amount of powder of 259 kg/hour was recycled through 3, the
content of Ca(OH).sub.2 being 10% by weight.
Through conduit 22 an amount of powder of 265 kg/hour with the said
content of non reacted calcium hydroxide was removed.
The temperature of the flue gas entering the spray drier was
154.degree. C. and the temperature of the effluent gas was
77.degree. C., which is about 24.degree. C. above its saturation
temperature.
The ratio between calcium hydroxide and sulfur dioxide was in this
case 2.5 and the absorption of sulfur dioxide 96%.
COMPARATIVE EXAMPLE 4
The procedure was the same in Example 3, except that the amount of
calcium hydroxide which in Example III was fed by recycling of
powder through 3 was replaced by a corresponding amount of fresh
calcium hydroxide. In this case the sulfur dioxide absorption was
80%.
EXAMPLE IV
A pilot plant of the type describes in Example I was used. The
amount of flue gas was in this case 20,800 kg/hour with a sulfur
dioxide content of 548 ppm. The flue gas had a fly ash content of
4.5 g/m.sup.3, the alcalinity of said fly ash being 0.89
milliequivalents per gram.
The temperature of the flue gas was 138.degree. C. when entering
the drier and 73.degree. C. when leaving it.
A suspension having a total solids content of 51.9% by weight was
fed to the spray drier. The concentration of freshly added lime was
4.1% while the remaining part of the solids was constituted by
recycled fly ash-containing powder.
A sulfur dioxide removal amounting to a total of 93.8% was
obtained. The stoichiometric ratio was calculated as 0.76.
It appears from this Example that the alkalinity of fly ash can be
utilized in the process according to the invention, as it can be
calculated that a proportion of 0.32 milliequivalents per g of the
fly ash alkalinity was utilized.
EXAMPLE IV
The pilot plant described in Example I was used under the following
conditions:
Amount of flue gas: 19,900 kg/hour
Temperature of inlet gas: 135.degree. C.
Temperature of outlet gas: 72.degree. C.
SO.sub.2 concentration: 170 ppm
Fly ash concentration: 4.5 mg/m.sup.3
Fly ash alkalinity: 0.84 milliequivalents per g
The dry matter concentration of the suspension to be atomized was
47.5% by weight. The content of freshly added lime in this
suspension was 0.9%.
A sulfur dioxide removal of 97.8% was obtained. The stoichiometric
ratio was calculated as 0.39.
It can be calculated that in this case a proporion of 0.43
milliequivalents/g of the fly ash alkalinity was utilized.
* * * * *